Anisotropic expansion of silicon-dominant anodes

ABSTRACT

Systems and methods for anisotropic expansion of silicon-dominant anodes may include a cathode, an electrolyte, and an anode, where the anode may include a current collector and an active material on the current collector. An expansion of the anode during operation may be configured by a thickness of the current collector. The expansion of the anode may be more anisotropic for thicker current collectors. A thicker current collector may be 10 μm thick or greater. The expansion of the anode may be more anisotropic for more rigid materials used for the current collector. A more rigid current collector may include nickel and a less rigid current collector may include copper. The expansion of the anode may be more anisotropic for a rougher surface current collector.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application is a divisional application of application Ser. No.16/674,224 filed on Nov. 5, 2019, which is hereby incorporated byreference in its entirety.

FIELD

Aspects of the present disclosure relate to energy generation andstorage. More specifically, certain embodiments of the disclosure relateto a method and system for anisotropic expansion of silicon-dominantanodes.

BACKGROUND

Conventional approaches for battery anodes may be costly, cumbersome,and/or inefficient—e.g., they may be complex and/or time consuming toimplement, and may limit battery lifetime.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with some aspects of the present disclosureas set forth in the remainder of the present application with referenceto the drawings.

BRIEF SUMMARY

A system and/or method for anisotropic expansion of silicon-dominantanodes, substantially as shown in and/or described in connection with atleast one of the figures, as set forth more completely in the claims.

These and other advantages, aspects and novel features of the presentdisclosure, as well as details of an illustrated embodiment thereof,will be more fully understood from the following description anddrawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram of a battery with configured anode expansion, inaccordance with an example embodiment of the disclosure.

FIG. 2 illustrates anode expansion during lithiation, in accordance withan example embodiment of the disclosure.

FIG. 3A illustrates roll press and flat press of anode active material,in an example embodiment of the disclosure.

FIG. 3B illustrates lateral expansion for roll press and flat presslamination anodes, in accordance with an example embodiment of thedisclosure.

FIG. 4 illustrates different foil surfaces for anode current collectors,in accordance with an example embodiment of the disclosure.

FIGS. 5A and 5B illustrate cycle life for cells with different currentcollectors and pyrolysis temperatures, in accordance with an exampleembodiment of the disclosure.

FIG. 6 illustrates roughened foils formed by etching, in accordance withan example embodiment of the disclosure.

FIG. 7 is a flow diagram of a process for more anisotropic expansion ina silicon anode, in accordance with an example embodiment of thedisclosure.

FIG. 8 is a flow diagram of a process for less anisotropic expansion ina silicon anode, in accordance with an example embodiment of thedisclosure.

DETAILED DESCRIPTION

FIG. 1 is a diagram of a battery with configured anode expansion, inaccordance with an example embodiment of the disclosure. Referring toFIG. 1 , there is shown a battery 100 comprising a separator 103sandwiched between an anode 101 and a cathode 105, with currentcollectors 107A and 107B. There is also shown a load 109 coupled to thebattery 100 illustrating instances when the battery 100 is in dischargemode. In this disclosure, the term “battery” may be used to indicate asingle electrochemical cell, a plurality of electrochemical cells formedinto a module, and/or a plurality of modules formed into a pack.

The development of portable electronic devices and electrification oftransportation drive the need for high performance electrochemicalenergy storage. Small-scale (<100 Wh) to large-scale (>10 KWh) devicesprimarily use lithium-ion (Li-ion) batteries over other rechargeablebattery chemistries due to their high-performance.

The anode 101 and cathode 105, along with the current collectors 107Aand 107B, may comprise the electrodes, which may comprise plates orfilms within, or containing, an electrolyte material, where the platesmay provide a physical barrier for containing the electrolyte as well asa conductive contact to external structures. In other embodiments, theanode/cathode plates are immersed in electrolyte while an outer casingprovides electrolyte containment. The anode 101 and cathode areelectrically coupled to the current collectors 107A and 1078, whichcomprise metal or other conductive material for providing electricalcontact to the electrodes as well as physical support for the activematerial in forming electrodes.

The configuration shown in FIG. 1 illustrates the battery 100 indischarge mode, whereas in a charging configuration, the load 107 may bereplaced with a charger to reverse the process. In one class ofbatteries, the separator 103 is generally a film material, made of anelectrically insulating polymer, for example, that prevents electronsfrom flowing from anode 101 to cathode 105, or vice versa, while beingporous enough to allow ions to pass through the separator 103.Typically, the separator 103, cathode 105, and anode 101 materials areindividually formed into sheets, films, or active material coated foils.Sheets of the cathode, separator and anode are subsequently stacked orrolled with the separator 103 separating the cathode 105 and anode 101to form the battery 100. In some embodiments, the separator 103 is asheet and generally utilizes winding methods and stacking in itsmanufacture. In these methods, the anodes, cathodes, and currentcollectors (e.g., electrodes) may comprise films.

In an example scenario, the battery 100 may comprise a solid, liquid, orgel electrolyte. The separator 103 preferably does not dissolve intypical battery electrolytes such as compositions that may comprise:Ethylene Carbonate (EC), Fluoroethylene Carbonate (FEC), PropyleneCarbonate (PC), Dimethyl Carbonate (DMC), Ethyl Methyl Carbonate (EMC),Diethyl Carbonate (DEC), etc. with dissolved LiBF₄, LiAsF₆, LiPF₆, andLiClO₄ etc. The separator 103 may be wet or soaked with a liquid or gelelectrolyte. In addition, in an example embodiment, the separator 103does not melt below about 100 to 120° C., and exhibits sufficientmechanical properties for battery applications. A battery, in operation,can experience expansion and contraction of the anode and/or thecathode. In an example embodiment, the separator 103 can expand andcontract by at least about 5 to 10% without failing, and may also beflexible.

The separator 103 may be sufficiently porous so that ions can passthrough the separator once wet with, for example, a liquid or gelelectrolyte. Alternatively (or additionally), the separator may absorbthe electrolyte through a gelling or other process even withoutsignificant porosity. The porosity of the separator 103 is alsogenerally not too porous to allow the anode 101 and cathode 105 totransfer electrons through the separator 103.

The anode 101 and cathode 105 comprise electrodes for the battery 100,providing electrical connections to the device for transfer ofelectrical charge in charge and discharge states. The anode 101 maycomprise silicon, carbon, or combinations of these materials, forexample. Typical anode electrodes comprise a carbon material thatincludes a current collector such as a copper sheet. Carbon is oftenused because it has excellent electrochemical properties and is alsoelectrically conductive. Anode electrodes currently used in rechargeablelithium-ion cells typically have a specific capacity of approximately200 milliamp hours per gram. Graphite, the active material used in mostlithium ion battery anodes, has a theoretical energy density of 372milliamp hours per gram (mAh/g). In comparison, silicon has a hightheoretical capacity of 4200 mAh/g. In order to increase volumetric andgravimetric energy density of lithium-ion batteries, silicon may be usedas the active material for the cathode or anode. Silicon anodes may beformed from silicon composites, with more than 50% silicon, for example.

In an example scenario, the anode 101 and cathode 105 store the ion usedfor separation of charge, such as lithium. In this example, theelectrolyte carries positively charged lithium ions from the anode 101to the cathode 105 in discharge mode, as shown in FIG. 1 for example,and vice versa through the separator 105 in charge mode. The movement ofthe lithium ions creates free electrons in the anode 101 which creates acharge at the positive current collector 1078. The electrical currentthen flows from the current collector through the load 109 to thenegative current collector 107A. The separator 103 blocks the flow ofelectrons inside the battery 100, allows the flow of lithium ions, andprevents direct contact between the electrodes.

While the battery 100 is discharging and providing an electric current,the anode 101 releases lithium ions to the cathode 105 via the separator103, generating a flow of electrons from one side to the other via thecoupled load 109. When the battery is being charged, the oppositehappens where lithium ions are released by the cathode 105 and receivedby the anode 101.

The materials selected for the anode 101 and cathode 105 are importantfor the reliability and energy density possible for the battery 100. Theenergy, power, cost, and safety of current Li-ion batteries need to beimproved in order to, for example, compete with internal combustionengine (ICE) technology and allow for the widespread adoption ofelectric vehicles (EVs). High energy density, high power density, andimproved safety of lithium-ion batteries are achieved with thedevelopment of high-capacity and high-voltage cathodes, high-capacityanodes and functionally non-flammable electrolytes with high voltagestability and interfacial compatibility with electrodes. In addition,materials with low toxicity are beneficial as battery materials toreduce process cost and promote consumer safety.

The performance of electrochemical electrodes, while dependent on manyfactors, is largely dependent on the robustness of electrical contactbetween electrode particles, as well as between the current collectorand the electrode particles. The electrical conductivity of siliconanode electrodes may be manipulated by incorporating conductiveadditives with different morphological properties. Carbon black(SuperP), vapor grown carbon fibers (VGCF), and a mixture of the twohave previously been incorporated separately into the anode electroderesulting in improved performance of the anode. The synergisticinteractions between the two carbon materials may facilitate electricalcontact throughout the large volume changes of the silicon anode duringcharge and discharge.

State-of-the-art lithium-ion batteries typically employ agraphite-dominant anode as an intercalation material for lithium.Silicon-dominant anodes, however, offer improvements compared tographite-dominant Li-ion batteries. Silicon exhibits both highergravimetric (3579 mAh/g vs. 372 mAh/g for graphite) and volumetriccapacities (2194 mAh/L vs. 890 mAh/L for graphite). In addition,silicon-based anodes have a lithiation/delithiation voltage plateau atabout 0.3-0.4V vs. Li/Li+, which allows it to maintain an open circuitpotential that avoids undesirable Li plating and dendrite formation.While silicon shows excellent electrochemical activity, achieving astable cycle life for silicon-based anodes is challenging due tosilicon's large volume changes during lithiation and delithiation.Silicon regions may lose electrical contact from the anode as largevolume changes coupled with its low electrical conductivity separate thesilicon from surrounding materials in the anode.

In addition, the large silicon volume changes exacerbate solidelectrolyte interphase (SEI) formation, which can further lead toelectrical isolation and, thus, capacity loss. Expansion and shrinkageof silicon particles upon charge-discharge cycling causes pulverizationof silicon particles, which increases their specific surface area. Asthe silicon surface area changes and increases during cycling, SEIrepeatedly breaks apart and reforms. The SEI thus continually builds uparound the pulverizing silicon regions during cycling into a thickelectronic and ionic insulating layer. This accumulating SEI increasesthe impedance of the electrode and reduces the electrode electrochemicalreactivity, which is detrimental to cycle life.

A solution to the expansion of anodes is to configure the expansion thatoccurs with lithiation to be anisotropic, such that the expansion occursin a desired direction. For example, if a cell can withstand someexpansion in the z-direction (thickness of the anode), then theexpansion may be configured to be minimized in the x- and y-directions.Conversely, if the cell can withstand lateral x- and y-directionexpansion but not z-direction expansion, the anode expansion can beconfigured to minimize z-direction (thickness) expansion. Anodeexpansion can be controlled with the current collector foil thickness,the foil strength, the type of active material lamination process, andthe roughness of the foils. This is described further with respect toFIGS. 2-7 .

FIG. 2 illustrates anode expansion during lithiation, in accordance withan example embodiment of the disclosure. Referring to FIG. 2 , there isshown a current collector 201, adhesive 203, and an active material 205.It should be noted that the adhesive 203 may or may not be presentdepending on the type of anode fabrication process utilized, as theadhesive is not necessarily present in a direct coating process. In anexample scenario, the active material 205 comprises silicon particles ina binder material and a solvent, the active material 205 being pyrolyzedto turn the binder into a glassy carbon that provides a structuralframework around the silicon particles and also provides electricalconductivity. The active material 205 may be coupled to the currentcollector 201 using the adhesive 203. The current collector 201 maycomprise a metal film, such as copper, nickel, or titanium, for example,although other conductive foils may be utilized depending on desiredtensile strength.

FIG. 2 also illustrates lithium particles impinging upon and lithiatingthe active material 205. The lithiation of silicon-dominant anodescauses expansion of the material, where horizontal expansion isrepresented by the x and y axes, whereas thickness expansion isrepresented by the z-axis, as shown. The current collector 201 has athickness t, where a thicker foil provides greater strength andproviding the adhesive 203 is strong enough, restricts expansion in thex- and y-directions, resulting in greater z-direction expansion, thusanisotropic expansion. Example thicker foils may be greater than 10 μmthick, 20 μm for copper, for example, while thinner foils may be lessthan 10 μm, such as 5-6 μm thick or less in copper.

In another example scenario, when the current collector 201 is thinner,on the order of 5-6 μm or less for a copper foil, for example, theactive material 205 may expand more easily in the x- and y-directions,although still even more easily in the z-direction without otherrestrictions in that direction. In this case, the expansion isanisotropic, but not as much as compared to the case of higher x-yconfinement.

In addition, different materials with different tensile strength may beutilized to configure the amount of expansion allowed in the x- andy-directions. For example, nickel is a more rigid, mechanically strongmetal for the current collector 201, and as a result, nickel currentcollectors confine x-y expansion when a strong enough adhesive is used.In this case, the expansion in the x- and y-directions may be morelimited, even when compared to a thicker copper foil, and result in morez-direction expansion, i.e., more anisotropic. In anodes formed with 5μm nickel foil current collectors, very low expansion and no crackingresults. Furthermore, different alloys of metals may be utilized toobtain desired thermal conductivity, electrical conductivity, andtensile strength, for example.

In an example scenario, in instances where adhesive is utilized, theadhesive 203 comprises a polymer such as polyimide (PI) orpolyamide-imide (PAI) that provides adhesive strength of the activematerial film 205 to the current collector 201 while still providingelectrical contact to the current collector 201. Other adhesives may beutilized depending on the desired strength, as long as they can provideadhesive strength with sufficient conductivity following processing. Ifthe adhesive 203 provides a stronger, more rigid bond, the expansion inthe x- and y-directions may be more restricted, assuming the currentcollector is also strong. Conversely, a more flexible and/or thickeradhesive may allow more x-y expansion, reducing the anisotropic natureof the anode expansion.

TABLE 1 illustrates x- and y-direction expansion of anodes withdifferent collector foil thickness and type of copper Material Thickness(μm) X-Expansion (%) Y-Expansion (%) C coated Cu 10 0.44 0.47 Cu A 101.64 1.57 Cu B 10 1.97 1.90 Cu C 10 1.55 1.48 Cu D 10 1.86 1.78 Cu E 63.30 3.01 Cu F 6 4.50 3.94 Cu G 6 2.00 1.70

As illustrated in the table, as the copper foil thickness decreases, theexpansion in the x- and y-directions increases and that adding a carboncoating on the copper foil may decrease expansion. This may be due tosurface roughening, as illustrated further with respect to FIGS. 4-6 .

FIG. 3A illustrates roll press and flat press of anode active material,in an example embodiment of the disclosure. Referring to FIG. 3A, thereis shown roll press lamination 310 comprising a current collector 301,active material 305, and rollers 307A and 307B. The current collector301 and the active material 305 may be similar to the current collector201 and active material 205 described with respect to FIG. 2 . Therollers 307A and 307B may comprise rigid cylindrical structures forapplying a configurable pressure to material passed between them in alamination process. It should be noted that while FIG. 3A shows activematerial on one side, the disclosure is not so limited, as the rollpress process applies to double-sided foils too.

Heat may be applied to the materials being laminated using heatingelements in the rollers 307A and 307B, or from external heat sources.Roll press lamination may result in significantly reduced x- andy-direction expansion as compared to flat press lamination 320 shown inthe inset of FIG. 3A. In flat press lamination, flat surfaces arepressed together to apply pressure to the electrode layers. Expansion ofanodes formed by roll press lamination 310 is compared to flat presslamination 320 in FIG. 3B.

The roll press lamination process thus has variables of pressure andtemperature, which can impact the anisotropic expansion ofsilicon-dominant anodes formed in this manner. For example, roll presslaminated anodes have higher anisotropic expansion (reduced x- andy-direction expansion, higher z-direction expansion) with lowertemperature and higher pressure during the roll press laminationprocess. In addition, the amount of solvent before, during, and afterlamination impacts the expansion of the layer, which may be tied to thetemperature employed during lamination. A higher amount of residualsolvent may remain before and during lamination for roll press. Afterlamination, there is no measurable difference in solvent residualbetween the roll press and flat press processes.

FIG. 3B illustrates lateral expansion for roll press and flat presslamination anodes, in accordance with an example embodiment of thedisclosure. Referring to FIG. 3B, there is shown x-direction andy-direction expansion of silicon-dominant anodes of various thicknessesand sources, where x and y-directions are illustrated in the inset abovethe plots. As can be seen, roll press laminated anodes demonstratesignificantly lower lateral (x- and y-direction) expansion as comparedto flat press laminated anodes. The boxed data points in each figure arefor otherwise identical anodes with 10 μm foil current collectors, butroll press laminated and flat press laminated, thereby demonstratingthat the lamination method has a strong influence on anode expansion.The roll-press laminated anodes demonstrate significantly reducedexpansion and no cracking down to 8 μm.

FIG. 4 illustrates different foil surfaces for anode current collectors,in accordance with an example embodiment of the disclosure. Referring toFIG. 4 , there is shown a foil 401 with a smooth surface 403 and a roughsurface 405. There is also shown silicon particles 407 from the anodeactive material. The size and shape of structures on the foil 401 aremerely for illustrative purposes, as surface roughness or siliconparticles may comprise any shape or size. In an example embodiment, therough surface 405 may comprise roughened foil material, such as a copperfoil with copper hills and valleys. In another example, the roughsurface 405 may comprise a coating such as carbon particles, carbonfibers, nanofibers, or rods for example, coated on the surface of thefoil 401.

The insets below the foil 401 in FIG. 4 are scanning electron microscope(SEM) images of the rough surface 405, where the particle sizes are onthe order of a few microns. Very strong adhesion may result in laminatedanodes due to the strong adhesion between the active material layer andthe foil with roughened surface. This bond allows the foil to helpconstrain the expansion of the active material layer. For anodes thatare directly coated onto a foil surface and then heat-treated(pyrolyzed), the contact is weaker, so a roughened copper helps with theadhesion by increasing surface area of the interface, which helps theexpanding active material layer be constrained by the foil.

In cells where expansion in the z-direction is less desirable than x-y,a smooth surface with roughness features smaller than a few microns, forexample, may be utilized thereby allowing more x-y expansion and lessz-direction expansion, illustrating how the anisotropic expansion of theanode active material may be configured.

FIGS. 5A and 5B illustrate cycle life for cells with different currentcollectors and pyrolysis temperatures, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 5A, there is shown cyclelife for anodes formed on a “plain” surface copper foil (not roughened).The silicon-dominant anodes use PAI as a binder and are direct coated oncopper foils, with some of them calendered, where a series of hardpressure rollers may be used to finish the film/substrate into asmoother and denser sheet of material, while others were not. As shownby the labels, some of the anodes are pyrolyzed at 550° C. and others at600° C. The cells with plain surface current collector all lost 40-60%of their capacity over 500 cycles.

Referring to FIG. 5B, there is shown cycle life for anodes formed onplain and roughened copper foil. The plain surface foil anodes with 550°C. pyrolysis correspond to similar cells from FIG. 5A, but the roughenedcopper foil cells show significantly improved cycle life, retaining85-95% of their capacity over 500 cycles.

Table 2 below shows x- and y-direction expansion for anodes withroughened copper foils after six formation cycles. For non-roughenedfoils, the average expansion is 2-3%, and as can be seen in the table,the expansion averages are in the 1.3-1.5% range for roughened foilanodes.

TABLE 2 X and Y Expansion of a Roughened Foil Surface Anode Anode X1 X2X3 Y1 Y2 1 1.48 1.41 1.28 1.28 1.27 2 1.54 1.39 1.31 1.24 1.34 3 1.501.41 1.34 1.31 1.34 4 1.58 1.56 1.48 1.28 1.33 5 1.65 1.55 1.42 1.371.49 6 1.44 1.47 1.37 1.36 1.37 7 1.56 1.47 1.33 1.33 1.41 8 1.55 1.451.32 1.35 1.41 9 1.54 1.47 1.33 1.26 1.44 10 1.45 1.45 1.29 1.20 1.29 111.39 1.38 1.26 1.27 1.33 12 1.44 1.37 1.23 1.07 1.22 AVG 1.51 1.45 1.331.27 1.35

FIG. 6 illustrates roughened foils formed by etching, in accordance withan example embodiment of the disclosure. Referring to FIG. 6 , there areshown foils 601A and 601B where material has been removed to form etchfeatures 603A and 603B. The etch features 603A and 603B create anartificial roughening of the surface by periodically removed copper, inthe example of a copper foil, leaving etch pits in the surface wheresilicon particles may embed. As shown with foil 601A, the etch features603A may be large enough that multiple silicon particles may embedwithin, as compared to foil 601B where the etch features 603B may belarge enough for just a single particle.

In an example scenario, the etch features 603A may range from 1 μmacross at the surface of the foil 601A to 50 μm, for example, where theD50 size of the silicon particles is on the order of 5-10 μm. The etchfeatures 603B may range from 5-15 μm, for example.

FIG. 7 is a flow diagram of a process for more anisotropic expansion ina silicon anode, in accordance with an example embodiment of thedisclosure. While one process to fabricate composite electrodescomprises physically mixing the active material, conductive additive,and binder together, and coating it directly on a current collector,this process employs a high-temperature pyrolysis process coupled with aroll pressing (calendering) process. This example process comprises adirect coating process in which an anode slurry is directly coated on acopper foil using a binder such as CMC, SBR, Sodium Alginate, PAI, PAA,PI and mixtures and combinations thereof. Another example processcomprises forming the active material on a substrate and thentransferring to the current collector. The process described here is forincreased anisotropy in anode expansion, where z-direction expansion isincreased while x-y expansion is decreased.

In step 701, a roughened foil may be obtained or fabricated by etchingthe surface. In addition, a thicker foil may be used to reduce the x-yexpansion of the anode during lithiation. Furthermore, a strongermaterial, such as nickel, may be used to further restrict lateralexpansion. In step 703, the raw electrode active material may be mixedusing a binder/resin (such as PI, PAI), solvent, and conductive carbon.For example, graphene/VGCF (1:1 by weight) may be dispersed in NMP undersonication for, e.g., 45-75 minutes followed by the addition of Super P(1:1:1 with VGCF and graphene) and additional sonication for, e.g., 1hour. Silicon powder with a 10-20 μm particle size, for example, maythen be dispersed in polyamic acid resin (15% solids in N-Methylpyrrolidone (NMP)) at, e.g., 800-1200 rpm for, e.g., 5-20 minutes, andthen the conjugated carbon/NMP slurry may be added and dispersed at,e.g., 1800-2200 rpm for, e.g., 5-20 minutes to achieve a slurryviscosity within 2000-4000 cP and a total solid content of about 30%.

In step 705, the slurry may be coated on the foil at a loading of, e.g.,3-4 mg/cm², which may undergo drying resulting in less than 15% residualsolvent content. In step 707, the foil and coating proceeds through aroll press for calendering. The pressure and temperature utilized duringroll press may increase the anisotropy of anode expansion.

In step 709, the active material may be pyrolyzed by heating to 500-800C such that carbon precursors are partially or completely converted intoglassy carbon. The pyrolysis step may result in an anode active materialhaving silicon content greater than or equal to 50% by weight, where theanode has been subjected to heating at or above 400 degrees Celsius.Pyrolysis can be done either in roll form or after punching in step 711.If done in roll form, the punching is performed after the pyrolysisprocess. The punched electrode may then be sandwiched with a separatorand cathode with electrolyte to form a cell. In step 713, the cell maybe subjected to a formation process, comprising initial charge anddischarge steps to lithiate the anode, with some residual lithiumremaining. The expansion of the anode may be measured to confirm thehighly anisotropic nature of the expansion, i.e., little x-y expansionand primarily z-direction expansion.

FIG. 8 is a flow diagram of a process for less anisotropic expansion ina silicon anode, in accordance with an example embodiment of thedisclosure. While the previous process to fabricate composite anodesphysically mixed the active material, conductive additive, and bindertogether and coated directly on a current collector, this processemploys a high-temperature pyrolysis process coupled with a flat presslamination process. After the raw electrode materials are mixed, theymay be coated on a substrate. The active layer may then be peeled intosheets, cut into desired size, cured, and undergo pyrolysis athigh-temperature to form an anode coupon with high Si content. The anodecoupon is then flat press laminated on an adhesive-coated currentcollector.

This process is shown in the flow diagram of FIG. 8 , starting with step801 where a thin metal foil, e.g., less than 10 μm, and comprisingcopper. In addition, the foil may be smooth, without any addedroughness, to allow x-y expansion. The active material may be mixed witha binder/resin such as polyimide (PI) or polyamide-imide (PAI), solvent,the silosilazane additive, and optionally a conductive carbon. In anexample scenario, silicon powder (5-20 μm particle size, for example)may be dispersed in NMP and silosilazane solution with the amount ofsilosilazane being 1.2% with respect to silicon. Polyamic acid resin(15% solids in NMP) may be added to the mixture at 500 rpm for 10minutes, and further dispersed between 700-1000 rpm for several hours toachieve a slurry viscosity within 1500-3000 cP (total solid content ofabout 30%).

In step 803, the slurry may be coated on a polymer substrate, such aspolyethylede terephthalate (PET), polypropylene (PP), or Mylar. Theslurry may be coated on the PET/PP/Mylar film at a loading of 3-4 mg/cm²(with 15% solvent content), and then dried to remove a portion of thesolvent in step 805. An optional calendering process nay be utilizedwhere a series of hard pressure rollers may be used to finish thefilm/substrate into a smoothed and denser sheet of material.

In step 807, the green film may then be removed from the PET, where theactive material may be peeled off the polymer substrate, the peelingprocess being optional for a polypropylene (PP) substrate, since PP canleave ˜2% char residue upon pyrolysis. The peeling may be followed by acure and pyrolysis step 809 where the film may be cut into sheets, andvacuum dried using a two-stage process (100-140° C. for 15 h, 200-240°C. for 5 h). The dry film may be thermally treated at 1000-1300° C. toconvert the polymer matrix into carbon. The pyrolysis step may result inan anode active material having silicon content greater than or equal to50% by weight, where the anode has been subjected to heating at or above400 degrees Celsius.

In step 811, the pyrolyzed material may be flat press laminated on thecurrent collector, where a thin copper foil, e.g., 6 μm or less, may becoated with polyamide-imide with a nominal loading of 0.3-0.6 mg/cm²(applied as a 6 wt % varnish in NMP, dried 10-20 hours at 100-120° C.under vacuum). The silicon-carbon composite film may be laminated to thecoated copper using a heated hydraulic press (30-90 seconds, 250-350°C., and 3000-5000 psi), thereby forming the finished silicon-compositeelectrode.

In step 813, the electrode may then be sandwiched with a separator andcathode with electrolyte to form a cell. The cell may be subjected to aformation process, comprising initial charge and discharge steps tolithiate the anode, with some residual lithium remaining. The expansionof the anode may be measured to confirm the less anisotropic (moreisotropic) nature of the expansion, i.e., allowed x-y expansion ascompared to the process of FIG. 7 . The thin and smooth foil and flatpress lamination result in more x-y expansion than anodes made fromthick and roughened foil with roll press lamination.

In an example embodiment of the disclosure, a method and system isdescribed for anisotropic expansion of silicon-dominant anodes. Thebattery may comprise a cathode, an electrolyte, and an anode, where theanode may comprise a current collector and an active material on thecurrent collector. An expansion of the anode during operation may beconfigured by a roughness of the current collector, a thickness of thecurrent collector, a metal used for the current collector, and/or alamination process that adheres the active material to the currentcollector.

The expansion of the anode may be more anisotropic for thicker currentcollectors. A thicker current collector may be 10 μm thick or greater.The expansion of the anode may be more anisotropic for more rigidmaterials used for the current collector. A more rigid current collectormay comprise nickel and a less rigid current collector may comprisecopper. The expansion of the anode may be more anisotropic for a roughersurface current collector. The roughness of the anode may be configuredby etching features in the current collector. The etched features mayrange from 1 to 50 μm across. The expansion of the anode may be moreanisotropic if the active material is roll press laminated to thecurrent collector. The expansion of the anode may be less anisotropic ifthe active material is flat press laminated to the current collector.

As utilized herein, “and/or” means any one or more of the items in thelist joined by “and/or”. As an example, “x and/or y” means any elementof the three-element set {(x), (y), (x, y)}. In other words, “x and/ory” means “one or both of x and y”. As another example, “x, y, and/or z”means any element of the seven-element set {(x), (y), (z), (x, y), (x,z), (y, z), (x, y, z)}. In other words, “x, y and/or z” means “one ormore of x, y and z”. As utilized herein, the term “exemplary” meansserving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.,” and “for example” set off lists ofone or more non-limiting examples, instances, or illustrations. Asutilized herein, a battery, circuitry or a device is “operable” toperform a function whenever the battery, circuitry or device comprisesthe necessary hardware and code (if any is necessary) or other elementsto perform the function, regardless of whether performance of thefunction is disabled or not enabled (e.g., by a user-configurablesetting, factory trim, configuration, etc.).

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments falling within the scope of the appended claims.

What is claimed is:
 1. A method of forming a battery, the methodcomprising: forming a battery comprising a cathode, an electrolyte, andan anode, the anode comprising a current collector and an activematerial on the current collector, the active material comprising apyrolyzed binder and 50% or greater silicon by weight; and controllingor adjusting an expansion of the anode during operation based on aselection of a thickness of the current collector so that the anodeexpands less in one of the x-y directions and the z-direction whileexpanding more in a different one of the x-y directions and thez-direction, wherein the thickness is selected, at least in part, basedon a material of the current collector.
 2. The method according to claim1, wherein the expansion of the anode is more anisotropic for the anodewith a thicker current collector with more anode thickness expansion andless lateral expansion as compared to an anode with a thinner currentcollector.
 3. The method according to claim 2, wherein the thickercurrent collector is 6 μm thick or greater.
 4. The method according toclaim 2, wherein the thicker current collector is 10 μm thick orgreater.
 5. The method according to claim 2, wherein the thicker currentcollector is 20 μm thick or greater.
 6. The method according to claim 1,wherein lateral expansion of the anode is less than 2% with a 10 μmthick current collector.
 7. The method according to claim 1, whereinlateral expansion of the anode is less than 5% with a 6 μm thick currentcollector.